1. Field of the Invention
The present invention relates to a hydraulic circuit for a power transmission device of a vehicle and more particularly, to improvement of the hydraulic circuit in which a discharge oil passage of an oil pump thereof is provided with an air bleeding orifice.
2. Description of Related Art
There has been known a hydraulic circuit of a power transmission device for a vehicle which includes (a) an oil pump for use as a hydraulic pressure source of hydraulic oil for actuating a hydraulic actuator of the power transmission device for the vehicle and (b) an air bleeding orifice which is provided so as to communicate with the discharge oil passage of the oil pump and discharge air in the discharge oil passage. Because hydraulic oil in the power transmission device is generally used as lubricant also, air is mixed during stirring at the time of lubrication. In addition, when the hydraulic oil is returned to an oil tank and used repeatedly, air trapping occurs in an intake oil passage and the discharge oil passage in the period of pump stop. Because this can obstruct rise-up of discharge pressure (oil pressure inside the discharge oil passage) at the initial period of the pump startup, an air bleeding orifice is provided to suppress occurrence of air trapping and at the same time, allow air to be discharged quickly at the initial period of the pump startup. In case where a mechanical pump and an electric oil pump are provided as oil pumps, there is a fear that at the time of the startup of the oil pumps, the rise-up of the discharge pressure may be delayed due to air in the oil passages thereby causing a shock or the like due to the operation delay of the hydraulic actuator of the power transmission device such as a gear ratio transmission. Thus, Japanese Patent Application Publication No. 2007-113640 (JP 2007-113640 A) has proposed provision of an air bleeding orifice in the discharge oil passage of the mechanical oil pump or the discharge oil passage of the electric oil pump.